Without limiting the scope of the present invention, its background is described with reference to pressure controlled well tools, as an example.
It is well known in the subterranean well drilling and formation testing arts that many types of well tools are responsive to pressure, either in the annulus or in the tool string. For example, different types of tools for performing drill stem testing operations are responsive to either tubing or annulus pressure, or to a differential therebetween. Additionally, other tools such as safety valves or drill string drain valves may be responsive to such a pressure differential.
Such well tools typically have some member, such as a piston, which moves in response to the selected pressure stimuli. Additionally, these well tools also typically have some mechanism to prevent movement of this member until a certain pressure threshold has been reached. For example, a piston may be either mechanically restrained by a mechanism such as shear pins or similar devices, whereby the pressure must exceed the shear value of the restraining shear pins for the member to move. Alternatively, a rupture disk designed to preclude fluid flow until a certain threshold pressure differential is reached may be placed in a passage between the movable member and the selected pressure source. Each of these techniques is well known to the art.
It has been found, however, that certain disadvantages exist where multiple pressure operated tools are utilized in a single tool string. In one conventional system for operating multiple tools in a tool string from the same pressure source, the operating pressures for the tool to be operated second are set at a pressures value greater than that required to operate the first tool. In some circumstances, this can present a disadvantage in that the releasing and operating pressure for the second-operated tool may be required to be higher than would be desirable. For example, in the above-stated example, it could be undesirable to apply the degree of pressure to the well annulus which might be necessary to operate the second-operated tool.
Therefore, a need has arisen for a well tool that is operable in response to a specific and predetermined pressure sequence in a variety of wellbore conditions. A need has also arisen for such a well tool that is operable to be selectively prevented from pressure related operations. Further, a need has arisen for such a well tool that is operable to be selectively enabled to responsive to pressure related operations.